FIGS. 12 to 15 are perspective views for explaining effects of quantum wire and quantum box structures described in IEEE Journal of Quantum Electronics, Vol. QE-22, No. 9, Sep. 1986, pp.1915.about.1921.
In FIG. 12, a GaAs bulk active layer 91 having a thickness exceeding 200.ANG. is sandwiched by upper and lower cladding layers 92 comprising Al.sub.0.2.Ga.sub.0.8 As. In FIG. 13, a GaAs quantum-film active layer 93 having a thickness of 100 .ANG. is sandwiched by upper and lower Al.sub.0.2 Ga.sub.0.8 As cladding layers 92. In FIG. 14, a 100.ANG..times.100.ANG. GaAs quantum wire 94 is buried in an Al.sub.0.2 Ga.sub.0.8 As cladding layer 92. In FIG. 15, a 100.ANG..times.100.ANG..times.100.ANG. cubic GaAs quantum box 95 is buried in an Al.sub.0.2 Ga.sub.0.8 As cladding layer 92. FIG. 16 is a graph illustrating quantum effects obtained in semiconductor lasers including the bulk layer 91, the quantum film 93, the quantum wire 94, and the quantum box 95, in which the ordinate shows obtained maximum gain and the abscissa shows injected carrier concentration.
A description is given of the principle and the operation.
When an active layer serving as a light emitting region of a semiconductor laser is surrounded by a material having a band gap energy larger than that of the active layer, injected charge carriers are confined in the active layer with high efficiency. In this description, it is assumed that the difference in the band gap energies is about 0.26 eV. When the thickness of the bulk active layer 91 shown in FIG. 12 is reduced to less than 200 .ANG., the quantum film 93 shown in FIG. 13 is obtained. The quantum film 93 provides larger gain than the bulk layer 91 even when the concentration of carriers injected into the active layer is the same. When this quantum effect is applied to the cross direction of the quantum film 93, the quantum wire 94 shown in FIG. 14 is obtained. Further, when the quantum effect is applied to the longitudinal direction of the quantum wire, the quantum box 95 shown in FIG. 15 is obtained. FIG. 16 is a graph illustrating calculated maximum gains of semiconductor lasers including the respective active layers 91, 93, 94, and 95 at different carrier concentrations. As shown in FIG. 16, when the carrier concentration is in a range of 3.about.4.times.10.sup.18 cm.sup.-3, the maximum gain increases in the order of the bulk 91, the quantum film 93, the quantum wire 94, and the quantum box 95.
Since the oscillation threshold current of the laser decreases with an increase in the gain, the threshold current decreases in the order of the bulk 91, the quantum film 93, the quantum wire 94, and the quantum box 95 in the above-described range of the carrier concentration.
At present, semiconductor lasers employing the bulk active layer 91 and the quantum film 93 have been put to practical use. However, the quantum wire 94 and the quantum box 95 are not put to practical use because of difficulty in fabrication.
However, the quantum wire 94 has been extensively studied recently, and semiconductor lasers including quantum wires have been manufactured by way of trial.
FIG. 17 is a sectional view illustrating a semiconductor laser including a quantum wire structure disclosed in, for example, Journal of Crystal Growth 93 (1988), pp.850.about.856.
In the figure, reference numeral 101 designates an n type GaAs substrate having a (100) surface orientation and a [011] oriented stripe-shaped V groove 109. Reference numeral 109a. designates a bottom of the groove 109. Reference numeral 109b designates inclined surfaces of the groove 109. Anon type Al.sub.0.5 Ga.sub.0.5 As lower cladding layer 121 having a thickness of 1.25 .mu.m is disposed on the surface of the GaAs substrate 101 including the V groove 109. An n type Al.sub.x Ga.sub.1-x As lower graded cladding layer 122 having a thickness of 0.2 .mu.m is disposed on the lower cladding layer 121, and the A1 composition ratio x of the lower graded cladding layer 122 gradually decreases upward from 0.5 to 0.2. A GaAs quantum active layer 123 having a thickness of 70.ANG. is disposed on the lower graded cladding layer 122. Reference numerals 123a and 123b designate portions of the quantum active layer 123 grown on the bottom 109a and the inclined surface 109b of the V groove 109, respectively. A p type Al.sub.x Ga.sub.1-x As upper graded cladding layer 124 having a thickness of 0.2 .mu.m is disposed on the active layer 123, and the al composition ratio x of the upper graded cladding layer gradually increases upward from 0.2 to 0.5. A p type Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 125 having a thickness of 1.25 .mu.m is disposed on the upper graded cladding layer 124. A p type GaAs cap layer 105 having a thickness of 0.2 .mu.m is disposed on the upper cladding layer 125. A p side electrode 107 is disposed on the cap layer 105, and an n side electrode 108 is disposed on the rear surface of the substrate 101. Reference numeral 106 designates current blocking regions produced by proton implantation.
A description is given of the production process.
Initially, the stripe-shaped V groove 109 extending in the [011] direction is produced at the (100) surface of the n type GaAs substrate 101 using an etchant comprising H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 (30 mole %): H.sub.2 O (volume ratio=1:8:40). The V groove 109 is=about 5 .mu.m wide at the opening and about 5 .mu.m deep.
Thereafter, the AlGaAs lower cladding layer 121, the AlGaAs lower graded cladding layer 122, the GaAs quantum active layer 123, the AlGaAs upper graded cladding layer 124, the AlGaAs upper cladding layer 125, and the GaAs cap layer 105 are successively grown on the surface of the substrate 101 having the groove 109 by MOCVD (Metal Organic Chemical Vapor Deposition). In this crystal growth, the respective growing AlGaAs layers maintain the shape of the V groove, and the portions of these layers grown on the inclined surface of the V groove are a little thicker than portions grown on the (100) surface of the substrate 101. The GaAs quantum-film active layer 123 has a relatively thick (about 100.ANG.) crescent-shaped portion 123a opposite the bottom 109a of the V groove. On the other hand, the active layer 123b grown on the inclined surface of the groove 109b is only 70.ANG. thick. Therefore, the active layer 123b has a band gap energy larger than that of the crescent shaped active layer 123a because of the quantum effect. In this-structure, the crescent-shaped active layer 123a is sandwiched between the upper and lower graded cladding layers 122 and 124 originally having band gap energies larger than that of the active layer 123 with respect to the perpendicular direction and, furthermore, the active layer 123a is sandwiched between the active layers 123b having band gap energies larger than that of the active layer 123a due to the difference in the thicknesses in the transverse direction, resulting in a quantum wire structure.
The difference in the band gap energies between the 100 .ANG. thick crescent-shaped active layer 123a and the 70.ANG. thick active layer 123b is only 0.023 eV, and this is smaller than the difference in the band gap energies between the cladding layer and the active layer, i.e., 0.26 eV, by one order of magnitude.
After the crystal growth process, proton ions are selectively implanted into the structure from the surface of the cap layer 105, excluding a region opposite the crescent-shaped active layer 123a, whereby current blocking regions 106 are produced. Thereafter, the p side electrode 107 is formed on the cap layer 105 and the n side electrode 108 is formed on the rear surface of the substrate 101 to complete the semiconductor laser shown in FIG. 18.
A description is given of the operation. When current is injected the p side electrode 107 and the n side electrode 108 which are connected to a positive electrode and a negative electrode of a current source, respectively, the injected current flows through a region opposite the bottom 109a of the V groove 109 where the current blocking regions 106 are absent and reaches into the quantum wire 123a, whereby laser oscillation occurs.
In the above-described prior art semiconductor laser, since the difference in band gap energies between the crescent-shaped active layer 123a and the active layers 123b on the inclined surfaces of the groove is very small, a satisfactory quantum effect is not obtained.
In order to improve the quantum effect, it is necessary to increase the difference in band gap energies in the transverse direction. In the prior art structure shown in FIG. 17, however, the difference in band gap energies in the transverse direction is produced utilizing the phenomenon that the active layer 123 grown in the V groove has a relatively thick portion 123a at the bottom of the groove and a relatively thin portion 123b at the inclined surface of the groove, which phenomenon is obtained under restricted conditions with little effect. Further, if the active layer 123a at the bottom of the groove is too thick, the quantum effect in the perpendicular direction is reduced. As the result, it is difficult to increase the difference in band gap energies of the active layer in the transverse direction by increasing the difference in thicknesses between the active layer 123a at the bottom of the groove and the active layer 123b on the inclined surface of the groove.
Furthermore, in the prior art structure, supposing that the same effect as described above is achieved when the stripe-shaped V groove is formed in the [011] direction, a quantum box structure can be produced by forming a reversed pyramidal recess on the substrate and growing an active layer on the substrate. In this case, however, for the same reasons as described above, it is difficult to increase the difference in band gap energies of the active layer in the transverse direction, so that a quantum box structure with improved quantum effect is not produced.